The specific heat of antiferromagnetic KMnF3 at low temperatures has been measured and the contribution of the Mn55 polarization extracted. The hyperfine field, when expressed as a frequency, is found to be A55Sh=686±10 Mc/sec. A55 has been separately measured for KMgF3: Mn2+, and the result is corrected to give A55 for KMnF3 at low temperature. By comparison of the two experiments, the ground-state spin alignment of the antiferromagnet is found to be SS=(99.8±1.5)%, in disagreement with spin-wave predictions. © 1962 The American Physical Society.